How do I remove Moon Phase calendar from Apple calendar?

When you say Moon Phase "calendar", do you mean just a set of events included in one of your regular calendars, or a calendar in its own right that's shown in the left sidebar? In the latter case, can't you just delete the calendar altogether?

You can change events from one calendar to another by clicking on the color patch drop-down menu on the right of the event title, in its details window.

If it's more complicated than that, post relevant pictures here. Press Shift-Command-5, frame a meaningful area, look in the options where the picture file will be saved (desktop by default), and press Enter. Add to the discussion with the second-last icon (mountain and moon) at the bottom of this text field.